The three different types of paleokarstification and reservoir distribution of Leikoupo Formation, Middle Triassic in the northern Sichuan Basin, China

• Autores: Xiong Ding, Bingwen Wang, Xingzhi Wang, Hong Liu, Deming Zeng, Minyou Feng
• Localización: Carbonates and Evaporites, ISSN 0891-2556, Vol. 33, Nº. 2, 2018, págs. 243-254
• Idioma: inglés
• Enlaces
  • Texto completo
• Resumen

  • The Middle Triassic Leikoupo Formation in the northern Sichuan Basin is a new discovery sequence for strategy replacement in the gas exploration field. The carbonate paleokarst reservoirs of the Middle Triassic Leikoupo Formation in the northern Sichuan Basin provide a case study of gas exploration in carbonate paleokarst reservoirs. By conducting a comprehensive study of well drilling, drill cores, well loggings, and test data, combined with the analysis of geological background of the Sichuan Basin, it is considered that syndepositional, supergene, and burial paleokarstification can be identified in the Leikoupo Formation, and these types of karstification were vital for reservoirs formation. Syndepositional karstification controlled the formation and distribution of early reservoirs. This type of karstification was limited to high-energy grain shoals, and therefore its horizontal extent was limited by the distribution of this facies. During temporary episodes of relative sea level fall, carbonate shoals were exposed subaerially or within the fresh water lens, and significant selective dissolution occurs, forming moldic pores and intragranular dissolved pores. The reservoirs related to supergene karstification mainly occurred in the area of the buried hill containing carbonate rock at the top of the Leikoupo Formation, and are vertically limited within a depth of 70–90 m under the carbonate corrosion unconformity plane of the Leikoupo Formation. The reservoirs related to supergene karstification formed mainly in limestone breccia and vuggy limestone in surface karst zone, and the high-angle dissolved fracture and medium- and small-scale corrosion holes in vertical vadose zone, and medium and small corrosion holes, dissolved fractures, roof fracture-caves in horizontal karst zone. The karst monadnocks within karst slope are the most favorable microgeomorphic units for horizontal distribution of reservoirs. Burial karstification was related to dissolution of carbonate minerals by organic acids, and was the key factor in optimal transformation of reservoirs. Burial karstification corroded and expanded the pore system left after infilling of the earlier pore system, and mainly was to optimize the transformation the reservoir space redistribution of earlier pore system. The compact carbonate layers related to burial karstification only can be dissolved along the early cracks in the channel and formation cracked reservoirs, but development of this type of reservoir was limited. The reservoirs were influenced by syndepositional, supergene, and burial paleokarstification in the Leikoupo Formation, and carbonate paleokarst reservoirs were formed finally.